The present invention relates generally to a zoom lens, and more particularly to a zoom lens which, albeit having a high zoom ratio, has a small telephoto ratio at the telephoto end, and is relatively simple in lens arrangement and reduced in size during collapsing.
In U.S. Pat. No. 4,773,744, the applicant has already proposed one of so-called zoom lenses for compact cameras. This zoom lens system comprises three lens groups having positive, positive and negative refracting powers in order from the object side, with each lens group moving toward the object side for zooming to a long focus side. The zoom lens system has the shortest length at the wide-angle end and the longest length at the telephoto end. This system is characterized in that very high magnifications are achievable by increasing the amount of movement of the three lens groups. Since focusing is carried out by moving out the second lens group, the most stable image formation capabilities are obtainable. For size reductions, numerous proposal have been offered to make the most of aspherical surfaces. Also, proposals have been offered to use a radial type of GRIN lenses on the same arrangement base.
For similar three-group zoom lens system, it has been attempted to allocate a substantial aberration correction role to aspherical surfaces, thereby reducing the number of lenses that form one lens group.
Even with the state of the art in development of aspherical surf aces, however, the amount of zooming movement of each lens group is still large. Development of the technology for ensuring image formation capabilities and making the total length of a lens system considerably short is virtually untapped at present. Nor any satisfactory result is still obtained.
Most of current zoom lenses for compact cameras have a mechanism for retracting a zoom lens in a camera body irrespective of their zooming types. To this end, a lens collapsing mechanism is provided in a space at the wide-angle end where the zoom lens is longest, there by reducing the space. When the camera is actually in use, on the other hand, the zoom lens is used in its original lens arrangement. In the case of a high-magnification zoom lens, a lens mount becomes very long because the amount of zooming movement becomes large on the telephoto side, and this is combined with a shift of the center-of-gravity position, resulting in a decentration problem.
In view of such problems as mentioned above, an object of the invention is to provide a zoom lens system which, albeit having a high zoom ratio, has a small telephoto ratio not only at its wide-angle end but also at its telephoto end, and is relatively simple in lens arrangement and reduced in size during lens collapsing.
In order to accomplish the aforesaid object, the present invention provides a zoom lens system comprising , in order from an object side thereof, a first lens group having positive refracting power, a second lens group having positive refracting power and a third lens group having negative refracting power, characterized in that:
for zooming from a wide-angle end to a telephoto end of said system, each lens group moves toward said object side while a spacing between said first lens group and said second lens group becomes wide and a spacing between said second lens group and said third lens group becomes narrow,
said first lens group comprises at least one set of a negative lens and a positive lens,
said second lens group includes an aperture stop and comprises at least one set of a negative lens and a positive lens,
said third lens group comprises one set of a positive lens and a negative lens, and
conditions (1), (2) and (3) are satisfied:
0.25 less than D2W/d1 less than 0.6xe2x80x83xe2x80x83(1)
0.03 less than |f3|/fT less than 0.25xe2x80x83xe2x80x83(2)
2.0 less than xcex23T/xcex23W less than 6.0xe2x80x83xe2x80x83(3)
where D2W is an actual air space between said second lens group and said third lens group at said wide-angle end, dl is a diagonal line length of an image plane of said zoom lens system that is a film or a diameter of an image circle, f3 is a focal length of said third lens group, fT is a focal length of said zoom lens system at said telephoto end, xcex23T is a transverse magnification of said third lens group at said telephoto end, and xcex23W is a transverse magnification of said third lens group at said wide-angle end.
It is here preferable that the first lens group consists of, in order from the object side of the system, one negative lens having reduced refracting power and one positive lens, the second lens group consists of, in order from the object side, one negative lens, a positive lens having a strong curvature on an object side thereof, an aperture stop and one positive lens having a strong curvature on an image side thereof, and the third lens group consists of one positive lens convex on an image side thereof and one negative lens strongly concave on an object side thereof.
Preferably, lenses that forms the first lens group having positive refracting power have at least two aspherical surfaces so as to make correction for spherical aberration in at least a telephoto region of the zoom lens system.
Preferably, a positive lens or a negative lens that forms a part of the second lens group having positive refracting power has at least one aspherical surface so as to make correction for at least off-axis aberrations.
Preferably, a negative lens that forms a part of the third lens group having negative refracting power has an aspherical surface on a strong concave surface thereof on an object side thereof so as to make correction for a peripheral image plane in at least a wide-angle region of the zoom lens system.
Preferably, a positive lens that forms a part of the third lens group has at least two aspherical surfaces on its surface strongly concave on its image side so as to make correction for a peripheral image plane in at least a wide-angle region of the zoom lens system.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.